1. Field of the Invention
The present invention relates generally to magnetic disk drives (disk drives), and more particularly to an efficient servo format that provides an increased servo sampling rate with minimal overhead.
2. Description of the Related Art
A conventional disk drive has a head disk assembly (HDA) including at least one disk, a spindle motor for rapidly rotating the disk, and a head stack assembly (HSA) for supporting a transducer head over the rotating disk. The servo control system uses xe2x80x9cservo informationxe2x80x9d recorded on the disk to position the head for reading and writing. The servo information most notably includes groups of magnetic transitions or xe2x80x9cburstsxe2x80x9d that are recorded in a radially displaced fashion within two angularly successive servo burst regions. In the simplest pattern of servo bursts, where only two bursts are used, they are usually designated as the xe2x80x9cAxe2x80x9d burst and the xe2x80x9cBxe2x80x9d burst. The radial displacement in such case places the xe2x80x9cAxe2x80x9d burst to one side of a burst pair centerline and the xe2x80x9cBxe2x80x9d burst to the other side. The xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d bursts are angularly displaced from one another since they are contained in angularly successive servo burst regions. Accordingly, the head passes over the xe2x80x9cAxe2x80x9d burst and then over the xe2x80x9cBxe2x80x9d burst. If the head is aligned with the burst pair centerline, then the head will pass over equal amounts of the xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d bursts and the servo control will develop a position error signal (PES) equal to zero. If the head is displaced from the centerline, then the head will pass over more of the xe2x80x9cAxe2x80x9d burst or over more of the xe2x80x9cBxe2x80x9d burst so that the PES will be nonzero, the sign of the PES indicating the direction of displacement. The servo information is most commonly embedded at equally spaced intervals in a data track and xe2x80x9csampledxe2x80x9d at a sampling rate which is determined by the spacing and rotation rate of the disk. The servo information occupies space, therefore, that could otherwise contain data.
One prior effort at reducing the spaced taken by the servo data is disclosed in U.S. Pat. No. 5,784,219 (""219 patent) authored by Genheimer, issued to Seagate Technology, Inc. on Jul. 21, 1998, and entitled DUAL SERVO FORMAT FOR OPTIMUM EFFICIENCY AND OFF-TRACK DETECTION. As suggested by its title, the ""219 patent discloses a xe2x80x9cdual servo formatxe2x80x9d of xe2x80x9clargexe2x80x9d servo wedges having four bursts (ABCD) and xe2x80x9csmallxe2x80x9d servo wedges having only two bursts (AB). The ""219 patent even taught that its small servo wedges might only contain the two bursts (AB), eliminating the remaining servo information normally found in a servo wedge (e.g. synchronization fields, track identification fields, and sector identification fields). The ""219 patent, however, was focused on defining a stand-alone burst pair centerline in its servo wedges, even its small ones. The ""219 patent""s small servo wedges, therefore, always include an xe2x80x9cAxe2x80x9d burst and a xe2x80x9cBxe2x80x9d burst that consumed data space.
The disk drive market is extremely competitive. Disk drive makers are continually striving for servo patterns that most effectively address the competing goals of increasing the available storage capacity (by providing less servo information) and enhancing performance (by providing more servo information). There remains a need, therefore, for a disk drive with a servo pattern that provides a more optimal solution to reducing the area occupied by the servo information while increasing the servo sampling frequency for purposes of shock protection, track following, or both.
In a first aspect, the invention may be regarded as a disk drive having a disk wherein the disk has a plurality of concentric tracks, each track comprising: a first servo sector; a second servo sector; and a first data region disposed between the first and second servo sectors; the first servo sector comprising: a synchronization field for establishing gain and frequency settings; a track identification field; and a plurality of servo bursts for position control; and the second servo sector comprising a single servo burst disposed in a predetermined alignment relative to one of the plurality of servo bursts in the first servo sector.
In a second aspect, the invention may be regarded as a method of more frequent servo sampling to improve shock performance while track following in a disk drive having a transducer and a disk with a plurality of concentric tracks wherein each track has a plurality of sequentially active interspersed first and second servo sectors wherein the servo sectors are separated by a data region, the method comprising the steps of: reading a track identification field from the first servo sector; reading a plurality of burst amplitudes from the first servo sector; calculating a position error signal from the track identification field and the plurality of burst amplitudes; providing a correction signal from the calculated position error signal; reading a single burst amplitude from the second servo sector; comparing the single burst amplitude to one of the plurality of burst amplitudes; and if the single burst amplitude differs by more than a predetermined threshold amount from the one of the plurality of burst amplitudes, aborting a current disk data transfer operation.